It's hard for human to understand this, but in 3D graphics objects have a singular reference point. So sometime objects may look in front, but it's ref point is behind another object. This distance will change and switch around depending on where you are in the scene. (change position) When this issue happens with a transparent object(steam), the drawing order can go wrong and appear to disappear. The Smoke is there, but the buidling is hiding it. The ref point cant be changed without causing other problems with other objects